Just ran across this and thought it might be of interest here. It's already been sold, but the pictures could be instructive to anyone contemplating a 4x4 bus build:
1997 International 4x4 School Bus 1874 - School Buses, Miscellaneous Buses - Buses for Sale
It was converted with Fabco parts which manufactures 4WD systems for medium and heavy duty trucks:
Fabco Automotive Corporation
I think the biggest takeaway from this is the lift need to fit a 4x4 front axle. Assuming a stock tire size of about 34", that means that the bottom edge of the nose (not the bumper) is about 34" off of the ground, as it seems to be even with the top of the tire. Compared to a stock International 3800, which seems to have the bottom edge of the nose situated about 3/4 of the way up the tire (about 25.5"), I think it's safe to say that about 10" is what is needed in order to fit an axle of sufficient strength under a full size bus. The Fabco axles are face loaders unlike military 2.5 ton units, so this is probably the minimum amount of lift one could get away with. Just food for thought...